Oops! Yes, title corrected. I don't know the history of the singing at Barcellona Pozzo di Gotto, except that it still goes on nowadays. The singers (all men) are called Visillanti, as their main task seems to be singing Vexilla Regis on Good Friday, in a sort of Sicilianised Latin ("Visilla Regis, &c."), very slowly with a great deal of ornamentation (improvised?), and with tremendous manly vigour, so far removed from our decorous British "college chapel" tradition.

I know Old Roman Chant only through the recordings by Marcel Peres & Ensemble Organum, who consciously (I believe) modelled their technique on traditional Sardinian male voice choirs who sound rather like the Visillanti, so whether the Sicilian chants are relics of ancient traditions or just, as you say, the popular music of the Mediterranean melting pot, I couldn't say.

One of the things I like about Marcel Peres, btw, is that for all the scholarship and artistic brilliance of his recordings, he still regards the chant as primarily liturgical music, and I understand he has a close working relationship with the Fraternity of St Peter.